The present invention relates to an image forming apparatus used in a copying machine and a printer.
Nowadays, in an image forming apparatus wherein a high speed and high image quality are required, there is employed, in most cases, an image forming method of an electrostatic latent image system with an electrophotographic system as a central system.
The reason for the foregoing is that an image with high image quality can be obtained at high speed and stably, and the aforementioned image forming method can be applied also to forming a color image and a digital image. It is therefore conceivable that the image forming method of an electrostatic latent image system may keep holding its great ground even in the future.
However, a level of demands from the field for the image forming technology is high, and it is going up year after year. Therefore, further progress of ability is demanded also in the electrostatic latent image system.
Among the aforesaid demands, the greatest one is further improvement of image quality as a matter of course, and as a measure for the demand, it is effective to make toner to be of a small particle size and to uniform the particle size distribution and particle shapes. However, it causes another problem, in many cases, to make toner to be a small particle and to uniform the particle distribution and particle shapes. Without taking any actions to these problems, practical use of them is difficult.
There has been a limit for enhancing a transfer rate without causing an image defect, and this is more tendentious when there are used toner of a small particle size and toner of uniformed distribution of particle sizes and uniformed particle shapes. If the transfer rate is lowered when a superposed color image is made by using plural photoreceptors and by transferring successively a monochrome image formed by each photoreceptor to superpose it, there are caused not only a fall of image density but also a disturbance of color balance, which is a serious problem. Further, for forming a color image, there are many modes wherein an intermediate transfer body is used, and transfer is conducted twice, including primary transfer to the intermediate transfer body and secondary transfer from the intermediate transfer body to the image support. In this case, an influence on the transfer rate is serious.
In order to avoid the aforementioned problems, it is necessary to find out a method wherein sufficient transfer rate can be obtained, even if a superposed color image is formed by transferring, plural times, a toner image made by small particle size toner in which particle size distribution and particle shapes are uniform, and even if the color images are transferred collectively.
The invention has been achieved to solve the aforementioned problems, and to further solve the following problems which cannot be ignored when developing a color image forming apparatus which is actually of a tandem type.
In the color image forming apparatus of a tandem type, it is necessary that plural toner images are transferred to be superposed on an intermediate transfer body or on an image support (both of these items are sometimes called collectively a transfer material), and when a toner image is further transferred newly on a toner image which has already been transferred on a transfer material, there must be selected a condition that the toner image to be transferred newly is transferred at a high transfer rate, and toner which has already been transferred is not transferred again onto the photoreceptor. Further, if a scratch is caused on an image forming body or an intermediate transfer body, toner tends to be stuck to the scratch, and its range is broadened gradually, resulting in toner filming. It is therefore important not to cause a scratch.
In most of transfer means for transferring toner images, there is used a transfer roller on which bias voltage is applied for transferring. In the case of forming images at high speed as in the present time, a value of an electric current for transfer per unit time tends to be high because transfer speed (moving speed) for a transfer material is also high and transfer time is short accordingly. Voltage that is necessary for transfer and is applied on a transfer roller is also high, and exfoliation discharge is generated when an image forming body leaves a transfer material, thus, transfer unevenness and toner repelling tend to be caused. It is therefore necessary to keep a value of an electric current for transfer per unit time down as far as possible even when the transfer time is short.
Further, owing to toner scattering or jamming of an image support, toner sometimes sticks to the transfer roller which, therefore, needs to be cleaned. As a cleaning method, if bias voltage which is opposite to that for transferring is applied to return toner to an intermediate transfer body or an image forming body once, and to collect the toner with a cleaning means, the transfer roller does not need to be provided with a cleaning means, which is preferable on the points of cost and space saving. However, for that purpose, characteristics of toner need to be improved so that reverse transfer may be carried out sufficiently.
Namely, objects of the invention are to keep the transfer efficiency for toner to be high for a long time and to find out measures for solving the following problems.
The first object is to find out conditions that a toner image to be transferred on a transfer material newly is transferred at the high transfer rate and another toner image which has already been transferred is not transferred again on an image forming body, when further transferring the toner image on another toner image which has already been transferred on a transfer material, and to provide an image forming apparatus wherein surfaces of an image forming body and of an intermediate transfer body are not scratched, and neither cleaning trouble nor toner filming is caused.
The second object is to provide an image forming apparatus wherein the process speed for transferring toner images is high, and high transfer efficiency can be obtained even when a value of an electric current to be impressed on a roller to prevent exfoliation discharge is lowered.
The third object is to provide an image forming apparatus wherein it is possible to return toner to an image carrier or to an intermediate transfer body by applying reverse bias voltage for the purpose of cleaning a transfer means, and to clean for certain.